1. Field of the Invention
The present invention relates to a deep drawing heat shrinkable multilayer film suitable for deep drawing top and bottom materials and the like and to a method of manufacturing the film.
2. Related Background Art
In conventional, stretchable and shrinkable multilayer films are widely used for packaging food such as raw meats, processed meat products, fishes, cheeses, and soups. As a method for packaging these contents, there are generally used a method for filling and packaging the contents into a bag or pouch bag, a method for filling and packaging the contents into a film of a bag just after the bag is formed by a vertical (longitudinal) pillow/transverse (horizontal) pillow type packaging machine, and a method for filling and packaging the contents by deep drawing.
For packaging the raw meats, a bag filling and packaging method is used in many cases. As such a bag filling and packaging method, a method is used in which a raw meat is filled in the bag manually and evacuated at a reduced pressure or vacuum evacuated in a vacuum chamber and thereafter an opening portion is sealed to provide a packaged product. In a series of operations in such filling and packaging methods, a filling rate is low. Accordingly an increase in the filling rate has been demanded from an economical point of view.
On the other hand, in a deep drawing packaging method with a filling rate higher than that of the bag filling and packaging method, unstretched and non-shrinkable multilayer films are used. Particularly, in the case where different types of content such as a ham, a roast pork, a bacon are filled and packaged, there are drawbacks that the packaged product is easily wrinkled due to poor film shrinkage, and that the films lack in a fitting property for the contents, so that liquid of the contents easily stays. Moreover, there is also a drawback in which film adhesion deteriorates due to poor film shrinkage, resulting in a poor keeping quality of contents.
In order to solve these problems, International Application Japanese-phase Publication No. 2003-535733 discloses a stretch-oriented multilayer film including at least three layers of: a surface layer (a) made of a thermoplastic resin; an intermediate layer (b) made of a polyamide-based resin; and a surface layer (c) made of a sealable resin. The multilayer film exhibits an impact energy of 1.5 Joule or more at a conversion thickness of 50 μm at a temperature of −10° C. In the specification, there is described a deep drawing package of a shrinkable multilayer film formed of a PET/mod-VL/Ny/EVOH/mod-VL/LLDPE resin structure.
However, in the conventional shrinkable multilayer film as described in JP 2003-535733 A, Ny and EVOH with an oxygen gas barrier property relating to the keeping quality of contents have humidity dependency. The above multilayer film has not always been satisfactory since, in particular, an oxygen gas transmission rate becomes high under high humidity, resulting in a poor oxygen gas barrier property under high humidity.